Among other pathophysiological processes, loss of adequate blood flow to part or all of the central nervous system (brain and spinal cord) and electrochemical irregularity in the central nervous system can cause sudden neurological dysfunction.
One example of sudden neurological dysfunction caused by the reduction of blood flow to the central nervous system is a cardiac arrhythmia that dyscoordinates heart contraction, reducing cardiac output and decreasing blood pressure. Arrhythmia of significant severity can lead to cessation of brain activity; if it is persistent and not promptly reversed, it can cause death. In emergency situations, the most commonly-employed means of reversing severe cardiac arrhythmia is defibrillation, a treatment that acts by delivering an electrical current to the heart. Electrodes applied to the chest or to the heart directly or through a transvascular approach are used to deliver electrical current to the heart. The patient is typically unresponsive at the time.
Another example of sudden neurological dysfunction caused by a blood flow abnormality is stroke, during which blood flow to the brain is disrupted either by occlusion of a cerebral artery (ischemic stroke) or by rupture of a cerebral artery (hemorrhagic stroke). Either type of stroke can cause sudden neurological dysfunction that is typically focal in nature and that often does not involve the loss or even impairment of consciousness or alertness. Ischemic stroke can be treated with intravenous enzymes that dissolve blood clots or by endovascular catheter devices that can physically disrupt or retrieve blood clots. Hemorrhagic stroke is typically treated using endovascular catheter devices or neurosurgical procedures to repair the ruptured artery.
An example of sudden neurological dysfunction that can be, but is not necessarily, triggered by a blood flow abnormality is seizure. During seizure, a portion or multiple portions of the central nervous system develop abnormal patterns of electrical activity, typically in which groups of neurons of the cortex become active in synchrony. Seizure in the emergency setting is typically treated with medications that act to sedate or desynchronize the activity of the abnormally-coordinated neurons.
By causing global ischemia of the central nervous system, cardiac arrhythmia can induce stroke and/or seizure activity in the acute setting by causing brain damage. Conversely, patients with focal brain damage from stroke or seizure can be unresponsive, and this can resemble cardiac dysfunction. The coincidence and symptomatic similarities of these conditions complicates the emergency diagnosis and treatment of a patient with sudden neurological dysfunction, particularly when one considers the worsened likelihood of survival and quality of clinical outcomes from delayed treatment of these time-dependent conditions.
Unlike cardiac arrhythmia, stroke and seizure are not known to be responsive to direct stimulation of the affected organ, i.e., the brain. However, selective stimulation of certain neural systems may influence stroke and seizure in a therapeutic manner. For example, some cranial and peripheral nerves (e.g., the trigeminal and facial nerves) appear to regulate the size of, and blood flow through, brain arteries. Also for example, some cranial and peripheral nerves (e.g., the vagal nerve) appear to regulate the excitability of neurons in the cerebral cortex that are prone to developing epileptiform activity consistent with seizures. In the latter case, an implantable vagal nerve stimulator has been studied and approved for use in the prevention of seizures in patients with epilepsy. To date, no stimulator device has been approved for the emergency treatment of stroke, although such devices are in development.